Currently only the four fixed-length SHA-3 algorithms are approved hash algorithms, providing alternatives to the SHA-2 family of hash functions. The XOFs can be specialized to hash functions, subject to additional security considerations. Guideline for using the XOFs will be provided in the future.

SHA-3 Derived Functions

In addition to four fixed-length hash functions, FIPS 202 also defines two eXtendable Output Functions, SHAKE128 and SHAKE256. Unlike the fixed-length hash functions, these SHAKE functions support variable-length outputs, and are named for their expected security level.

FIPS 202 also supports a flexible scheme for domain separation between different functions derived from KECCAK, ensuring that different named functions will produce unrelated outputs. NIST extends this flexibility further to allow users to customize their use of the functions by defining a customizable version of SHAKE, called cSHAKE.